1. Field of the Invention
The present invention relates to an image pickup apparatus such as a digital still camera, with which a lens is integrated. The present invention relates also to a camera body to which an interchangeable lens unit can be attached detachably. The present invention relates also to an interchangeable lens unit that can be attached detachably to a camera body. Further, the present invention relates to an ultrasonic motor driving apparatus for driving and controlling an ultrasonic motor.
2. Description of Related Art
Recently, digital single-lens reflex cameras rapidly have come into widespread use. In an observation of a photographic subject by using an optical finder of such a digital single-lens reflex camera, a light beam entering an image pickup optical system is reflected by a mirror disposed on an optical path and guided to a finder optical system. As a result, a subject image is converted into an erected image through a pentaprism or the like and guided to the optical finder. Thereby, through the optical finder, a photographer can observe a subject image formed by the image pickup optical system. As mentioned above, a mirror typically is disposed on an optical path.
In a typical digital single-lens reflex camera, a focus lens and a focus motor are loaded in order to focus the subject image on an image pickup surface of an image pickup element. In general, the focus motor is provided as a DC motor.
However, it is difficult to move the focus lens precisely with the DC motor. Moreover, the DC motor may cause noise. In order to cope with this problem, Patent document 1 (JP H11-356071 A) discloses a configuration for loading an ultrasonic motor that has excellent precision in moving a focus lens and noise reduction.
There is unevenness among the ultrasonic motors in the frequency characteristic (i.e., a characteristic representing a relationship between a drive frequency inputted into an ultrasonic motor and an output rotation number (rotation speed) of the ultrasonic motor). For checking the frequency characteristic, according to the configurations as disclosed in Patent document 2 (JP 2003-219668 A) and Patent document 3 (JP 2005-124296 A), cameras are actuated under a specific condition so as to measure the frequency characteristic of every product, and the frequency characteristic is written on the memory in the interchangeable lens. At the time a user turns on the camera in use, the user reads the frequency characteristic written on the memory and controls the drive of the ultrasonic motor with reference to the frequency characteristic.
However, even though the frequency characteristic is measured at the time of factory shipment of the camera, after a long-time use of the camera by the user, the mechanical components such as gear and cam each coupled to the ultrasonic motor and/or the components inside the ultrasonic motor are worn and the frequency characteristic of the ultrasonic motor will vary. For this reason, it is impossible to maintain a stable drive for a long time with the setting of the frequency characteristic at the time of factory shipment.